Synthetic pathways for selective introduction of a dicobalt hexacarbonyl cluster into a polyfunctional molecule: Methotrexate

Crystal structures of (μ2-η2,η2-4-hydroxybut-2-yn-1-yl 2-bromo-2-methylpropanoate-κ4C2,C3:C2,C3)bis[tricarbonylcobalt(II)](Co—Co) and [μ2-η2,η2-but-2-yne-1,4-diyl bis(2-bromo-2-methylpropanoate)-κ4C2,C3:C2,C3]bis[tricarbonylcobalt(II)](Co—Co)

The structures of two alkyne-hexacarbonyl-dicobalt complexes are reported, each with potential ATRP initiator substrates as substituents on the alkynes. The complexes each form tetrahedral C₂Co₂ cluster cores with classical sawhorse conformations, while a feature of the crystal packing is the formation of inversion dimers for both molecules.

The title compounds, [Co₂(C₈H₁₁BrO₃)(CO)₆], (1), and [Co₂(C₁₂H₁₆Br₂O₄)(CO)₆], (2), result from the replacement of two carbonyl ligands from dicobalt octa­carbonyl by the alkynes 4-hy­droxy­but-2-ynyl 2-bromo-2-methyl­propano­ate and but-2-yne-1,4-diyl bis­(2-bromo-2-methyl­propano­ate), respectively. Both mol­ecules have classic tetra­hedral C₂Co₂ cluster cores with the Co^II atoms in a highly distorted octa­hedral coordination geometry. The alkyne ligands both adopt a cis-bent conformation on coordination. In the crystal structure of (1), classical O—H⋯O and non-classical C—H⋯O contacts form inversion dimers. These combine with weak O⋯O and Br⋯O contacts to stack the mol­ecules into inter­connected columns along the b-axis direction. C—H⋯O and C—H⋯Br contacts stabilize the packing for (2), and a weak Br⋯O contact is also observed. Inter­connected columns of mol­ecules again form along the b-axis direction.